Fundamentals of Radiation

Partial Periodic Table

AlkaliMetals

1A

6C

12.01Carbon

AtomicChemical

AtomicName of

NumberSymbolMassElement

Noble Gases0

1H

1.01Hydrogen

AlkalineEarth

MetalsIIA IIIA IVA VA VIA VIIA

2He

4.00Helium

3Li

6.94Lithium

4Be

9.01Beryllium

5B

10.81Boron

6C

12.01Carbon

7N

14.01Nitrogen

8O

16.00Oxygen

9F

19.00Flourine

10Ne

20.18Neon

The Periodic Table provides the atomic number (Z), the chemical symbol, atomic mass, and element name. It also groups the elements based on their electron structure (I.e., how they react chemically).

Structure of the Atom

Nucleus

Orbiting Electrons

The nucleus contains neutrons and protons,

also referred to as nucleons. The electrons

orbit the nucleus.

The protons have a positive charge, the electrons a negative

charge, and the neutrons are not charged.

The electrons are responsible for chemical

reactions (e.g., formation of molecules).

The nucleons are responsible for nuclear

reactions (e.g., radioactive decay).

Nomenclature

Z = Number of Protons (determines the chemical element)

N = Number of Neutrons (determines the isotope of the

element)

A = Neutrons plus Protons (atomic mass of the isotope)

XA

Z NX = Chemical Symbol

A = Z + N

The chemical symbol and the atomic mass define the individual nuclide. (e.g., 3H has 1 proton and 2 neutrons).

Isotopes of an element have the same number of protons, but a different number of neutrons in the nucleus.

Forces in a Nucleus

Hydrogen-3 (Tritium) Helium - 3

p

n

n

Nuclear force of neutron on proton

Nuclear force of proton on neutron

p

n

p

Nuclear force of proton on proton

Nuclear force of proton on proton

Electrostatic force of proton on proton

Electrostatic force of proton on proton

Nuclear force is an attractive force between each of the nucleons (i.e., neutrons and protons) over relatively short distances.

Electrostatic force is a repulsive force between the like charged protons over a greater distance than nuclear forces.

Radioactive DecayThe nuclides, as with most things in nature, want to be at their lowest energy state which is a stable nucleus.

Radioactive decay occurs in nuclides where the nucleus is unstable.

For stable nuclides with low atomic masses, the number of neutrons is equal to, or approximately equal to the number of protons (except for 1H which only has one nucleon).

As the atomic mass of the nuclide increases, the ratio of neutrons to protons must be greater than one for it to be stable, suggesting that more neutrons are required to provide nuclear forces to offset the electrostatic repulsive force between the increased number of protons.

The nucleus may also become unstable when energy is added to it, placing it in an excited state. An example of this would be a free moving neutron inside of a reactor being captured by the nucleus of a 238U nucleus.

The nuclide reaches its stable state by undergoing radioactive decay.

Types of Radiation

There are four types of radiation of interest:1) Alpha () which is a positively charged helium

nucleus (2 protons and 2 neutrons).2) Beta () which is a negatively charged electron.3) Gamma () which is a packet of energy with zero

rest mass.4) Neutron (n) which is a released neutron. Mainly a

concern during nuclear reactor operation.

Alpha Particle

Helium-4 Nucleus(2 neutrons, 2 protons)

Slow moving, but high energyCannot penetrate material easilyStopped by one piece of paperStopped by dead layer of skin

Example of Alpha Decay

Alpha particle

Radium-226 Radon-222

(88 protons, 138 neutrons) (86 protons, 136 neutrons)

Alpha decay occurs when the nuclides of high atomic mass have a lower neutron to proton ratio than stable nuclides and ejects an alpha particle.

Alpha decay is rare for nuclides with low or intermediate mass numbers.

Beta Particle

ElectronFast moving, Medium energyCan penetrate material wellStopped by 100 to 150 pieces of paperStopped by 0.5 -1 centimeter of water

Example of Beta Decay

Beta particle

Carbon - 14 Nitrogen - 14(6 protons, 8 neutrons) (7 protons, 7 neutrons)

Beta decay occurs when the nuclides have a higher neutron to proton ratio than stable nuclides.

A neutron converts to a proton, electron (), and a neutrino.

A neutrino is a high energy particle with zero rest mass with high penetrating capability.

Gamma Decay

Electromagnetic radiationSimilar to light, x-rays, radio waves

Emitted only by certain nucleiSpeed of light; low to high energyHighly penetratingStop half of the s with about 1 cm of

leador 5 to 15 cm of water

Example of Gamma Decay

238U + neutron 239U +

Gamma decay occurs as a means of removing energy from the nucleus of an excited nuclide.

The gamma may be ejected alone or in conjunction with the emission of another radioactive particle (e.g., ) to reduce the nucleus energy.

Examples of Neutron Emission

There are Neutron (n) emissions associated with the following reactions.

2H + 1H + neutron9Be + 2 (4He) + neutron

9Be + 12C + neutronNeutron emissions of interest in a nuclear reactor occur when the excited nucleus of a specific high atomic mass nuclide

splits into two or more smaller nuclides during the fission process.

235U + n 135I + 97Y + 3nNeutrons with high kinetic energy are released in the process.

Half-life

Each radioactive nucleus has a certain probability of decay per timeSome decay quickly (fractions of a second), some later (thousands of years)Rate of decay depends on the number of nuclei availableAs number decreases, rate of decay decreases

Half-life

In theory, all the radioactive material will never totally decayDefine Half-lifeTime for half of the sample to decay

Half-lifeFra

ctio

n o

f th

e o

rig

inal

Time (Number of half-lives)

1

1

2

1

4

1

81

16

1 2 3 4

Example Half-lives - Natural

Uranium-238 (In soil)4.5 Billion years

Potassium-40 (in soil and body)1.3 Billion years

Carbon-14 (In all living tissue)5730 years

Hydrogen-3 (in all water)12 years

Example Half-lives - Natural

Radium-226 (In soil - produces radon)1600 years

Radon-222 (in soil and air)3.8 days

Polonium-214 (radon progeny that decays in lungs)164 microseconds (0.000164 s)

Example Half-lives - Medical Uses

Iodine - 131 (Thyroid treatment)8 days

Technetium-99m (Nuclear medicine)6 hours

Gold-198 (Tumor therapy)2.7 days

Activity

Activity = Decays per timeUnits:1 Becquerel = 1 decay per second (dps)1 Curie = 37 Billion dps1 microCurie (Ci) = 37,000 dps1 picoCurie (pCi) = 0.037 dps

Example Activities - Regulations

Typical maximum alpha emitting radionuclide allowed without a license (some exceptions)0.1 Ci

Typical maximum beta emitting radionuclide allowed without a license (some exceptions)10 Ci

Example Activities - Natural Radioactivity

Uranium-238 in cup of soil (typical)0.003 Ci = 3000 pCi

Radon-222 in air0.5 pCi per liter (outdoor air)1 to hundreds of pCi per liter (houses)

Potassium-40 in human body0.1 Ci

Radiation Dose

Dose = Energy absorbed per massUnits:Rad Gray (Gy) [1 Gy = 100 rad)

Radiation Dose Equivalent

Different radiations do different amounts of biological damageDose Equivalent = Dose X QFQF = Quality factorBetas, Gamma: QF = 1; Alpha: QF = 20

UnitsRem (1 mrem = 0.001 rem)Sievert (Sv) [1 Sv = 100 rem)]

Radiation Exposure

Old unit of exposureAmount of radiation present in airOnly applicable for x-rays and gamma

radiation

Units:Roentgen (R)1 R exposure in air will produce about

1 rad dose in human tissue

Example Doses

Natural annual background radiationcosmic: 27 mrem (0.27 mSv)Terrestrial: 28 mrem (0.28 mSv)Internal: 39 mrem (.039 mSv)

[total natural (excl. Radon): ~100 mrem]

Radon-lungs: 2400 mrem (24 mSv) [effective whole body: 200 mrem]

Source: NCRP Report #93

Example Doses

Medical Radiation (Effective Whole Body Dose Equivalent)Chest X-ray: 8 mrem (0.08 mSv)Head CT scan: 111 mrem (1.11 mSv)Barium Enema: 406 mrem (4.06 mSv)Extremity X-ray: 1 mrem (0.01 mSv)

Source: NCRP Report 100

Radiation Safety

Radioactive materials produce a dose rate per timeTo reduce total dose:Minimum time

Half the time - half the dose

ShieldingMaximum distance

Twice the distance - one-fourth the dose